Vaginal Lock and Key

Male and female snake genitalia have evolved together in a remarkable example of lock and key compatibility.

The patterns are striking: - Species with branched hemipenes have females with branched cloacas - Species with spiny hemipenes have females with thicker cloacal walls - The shapes and structures match within each species

This co-evolution ensures that snake mating is species-specific. Even closely related species that look similar externally may have incompatible genitalia that prevent hybridization.

Scientists call this sexual selection through genital evolution. It works both ways: - Males evolve structures that fit particular female anatomy - Females evolve anatomy that accepts only certain male structures - Over time, genitalia become increasingly specialized

The spines on male hemipenes are particularly interesting. They're not weapons — they appear to stimulate females during mating and help males stay connected during copulation.

Females with thicker vaginal walls in spiny-penis species may have evolved: - As protection from damage - To increase stimulation from the spines - To better "filter" between males

This genital arms race happens across many animal groups, but snakes show some of the clearest examples because: - They have well-documented hemipenis variation - Many similar species live in overlapping ranges - Genital differences often define species boundaries

When you can't tell two snakes apart by looking at them, examining their genitalia often reveals they're different species entirely.